Mycobacterium avium complex

Ziekteverwekker

Avian tuberculosis is a disease in birds caused by bacteria of the genus Mycobacterium. There are many Mycobacterium species. They are grouped into different complexes and clades [7]. Not all species are pathogens. From a medical perspective, the species belonging to the M. tuberculosis complex and the M. avium complex are of particular importance. Species from the M. tuberculosis complex mainly infect mammals, and species from the M. avium complex mainly infect birds [2].

Avian tuberculosis is usually caused by species from the M. avium complex, and sometimes by Mycobacterium species from other complexes and clades. In 1975-1985, wild birds were examined post-mortem in the Netherlands [6]. From the 82 Mycobacterium-positive birds, six different species were cultured: 76 M. avium, 4 M. fortuitium, 1 M. terrae, one M. nonchromogenicum, 1 M. flavescens, and an unspecified Mycobacterium species (2 cases of co-infection, i.e., simultaneous infection by different Mycobacterium species).

Gevoelige diersoorten

M. avium complex can also cause disease in humans, particularly in AIDS patients.

M. avium complex infection has also been found in domesticated and wild mammals, such as pigs, rabbits, mink, sheep, and deer [1].

Avian tuberculosis is a disease that is widespread worldwide. All bird species appear to be susceptible, albeit to varying degrees. Avian tuberculosis is most frequently seen in small hobby farms, zoos, and wild birds.

Mycobacterium spp. was observed in 82/11,664 (0.7%) of the cases examined in 1875-1985 [6]. It has been found in 25 bird species, from 10 different orders. However, according to the authors, some strains were only incidental and their pathological significance remained undetermined. Similar prevalences have been observed in the United States and Canada: studies found Mycobacterium spp. infection in 0.3% of 3,000 waterfowl and 0.6% of 600 wild birds, respectively [1].

Bacteria are excreted in feces by infected birds with advanced lesions. Carcasses and slaughter waste can infect predators or cannibalistic birds. Recent molecular studies [5] indicate heterogeneity within the M. avium complex, and the epidemiology and pathogenesis may vary between species.

Symptomen dieren

Avian tuberculosis is a slowly spreading, chronic, granulomatous disease characterized by gradual weight loss in the infected bird.

M. avium leads to a widespread infection, in which the liver, spleen, intestines, and bone marrow are primarily affected in domesticated poultry. Granulomatous lesions of varying sizes are found in these organs [4, 8], which are rarely calcified, unlike M. tuberculosis infection lesions [4]. Clinical symptoms usually develop late; the birds lose weight and lameness may be observed.

The preliminary diagnosis is made based on large numbers of acid-fast bacteria in organ impressions. The diagnosis is confirmed by laboratory examination [7].

Voorzorgsmaatregelen

In commercial poultry, this previously common infection has largely disappeared due to the short lifespan of the animals combined with improved sanitation measures. In the event of infection, the poultry must be destroyed, as treatment is ineffective and not all infected birds can be detected with current tests [1]. Housing must also be thoroughly cleaned and disinfected. M. avium is highly resistant; it can survive in the soil for several years. Contaminated soil on farms must be removed by a few centimeters and replaced [2].

It is important to limit the spread of avian tuberculosis caused by humans as much as possible, including in nature conservation activities. For example, if birds of prey in rescue centers are fed carcasses of wild birds, these carcasses must be checked for avian tuberculosis before feeding to prevent infected birds of prey from being released [1]. This is important because infected birds appear clinically healthy and results from diagnostic tests in live birds are not reliable. Eradication of the infection in wild bird populations is not possible. In wetlands, habitat manipulation (e.g., drainage and bird deterrents) is used to keep wild birds away from locations where repeated outbreaks have occurred. The idea is to prevent healthy birds from becoming infected by the bacterium, which would be present in large quantities in the environment as a result of the outbreaks. Because the bacterium can survive in the environment for a long time, such measures are generally taken for about 2 years [1].

The persistence of avian tuberculosis in zoo collections attests to the difficulty of controlling the disease [1]. Newly introduced birds must be quarantined for 2-3 months [3, 5].

References

  1. Friend, M., Franson, J. C., & Ciganovich, E. A. (Eds.). (1999). Field manual of wildlife diseases: general field procedures and diseases of birds. Geological Survey (USGS). p. 93-98
  2. 2010. Merck Veterinary Manual. 10th ed. Publisher, Whitehouse Station, N.J.: Merck & Co. Editor: Cynthia M. Kahn. 2945 pp.
  3. Cooper J.E. 2007. Birds of prey: health & disease. 3rd ed. Wiley-Blackwell. 364 pp.
  4. Fulton R.M. & Thoen C.O. 2003. In: Diseases of poultry. 11th ed. Saif Y.M. Blackwell Press. 1233 pp.
  5. Schrenzel M. Nicolas M. Witte C. Papendick R. Tucker T. Keener L. Sutherland-Smith M. Lamberski N. Orndorff D. Heckard D. Witman P. Mace M. Rimlinger D. Reed S. and Rideout B. 2008. Molecular epidemiology of Mycobacterium avium subsp. avium and Mycobacterium intracellulare in captive birds. Veterinary microbiology, 126, 1-3: 122-131.
  6. Smit T. Eger A. Haagsma J. and Bakhuizen T. 1987. http://www.jwildlifedis.org/doi/pdf/10.7589/0090-3558-23.3.485 Avian Tuberculosis in Wild Birds in the Netherlands. Journal of Wildlife Diseases, 23, 3: 485-487.
  7. Turenne C.Y. Wallace R. and Behr M.A. 2007. Mycobacterium avium in the Postgenomic Era. Clinical microbiology reviews, 20, 2: 205-229.
  8. Whiteman C.E. & Bickford AA. 1989. Avian disease manual. 3rd ed. Dubuque, Iowa: Kendall/Hunt. 242 pp.